I'm 16 and have just about finished school. In November 2011, I received money from my father for necessities, shopping and costs for a college course. On 26 November, I lost my bank card so decided to cancel it as there was £2,992.99 left in there and the cost of the course is £1,999.99. I decided not to touch my account until nearer the date of the course.

I received an online banking statement in May and 41p was left in the account. Transactions had left my account at places I have never been to or don't need to go to. I have contacted Santander, but it is refusing to refund the amount, as the card should have been cancelled. Without this money I can't go to college. GA, east London

This turned out to be a lot more serious than it first appeared. You claim you had £2,992 in your account before anything happened, but Santander says the balance was showing £9 overdrawn. Then, between 25 and 28 November, four payments (which turned out to be fraudulent) totalling £4,802.13 began crediting your account from another Santander customer's account. After unusual spending on the card, Santander blocked it, but the bank then said you made "repeated calls", cleared security, checked the balance and had the block removed. On 28 November, when the money that went in had been spent and there was only £500 left, you again confirmed to Santander by phone that the previous transactions were genuine. This makes no sense, given that the transfers in were fraudulent and (you now claim) transfers out were not made by you. On the same day, you also reported your card had been lost on the morning of 26 November.

On 11 January 2012, Santander sent you a letter telling you that your account was to be closed because it had received fraudulent transactions, but you did not contact the bank until May, when you finally disputed the four transfers, claiming you had reported the card stolen. But Santander confirmed the fraud began before you reported the card had been lost.

By this time, Santander suspected "mule fraud" – and I agree with them. Money mules receive funds (the proceeds of fraud on other accounts) into their accounts, which they then withdraw and pass back to the fraudsters. Santander suspects you were probably promised a certain sum for helping out. I suspect you contacted the Observer when you realised you had been left less than promised and thought we could claw it back.

You say you have not willingly taken part in a scam and were expecting money to pay for your education from your father, which is why you didn't think £4,000-plus entering your account was strange. But this does not explain why you initially told the bank the fraudulent transactions were genuine, nor does it explain why you believe you have been left out of pocket – you were £9 overdrawn before it kicked off.

Financial Fraud Action UK says criminals looking to recruit a money mule often target vulnerable groups such as university students tempted by the lure of an apparently easy way of making extra cash. When caught, mules can suffer severe penalties, including a prison sentence of up to 10 years. Any profits they have made will be recovered from their accounts to reimburse the victims of fraud, while their bank account will be closed down and a bank may even share details of the activity with other banks (meaning they may no longer be able to open a bank account in the UK).

Santander has forcibly closed your account and you are no longer a customer, but it has not passed details of what happened to the police and you say you have managed to open an account with another bank.

You can email Mark King at your.problems@observer.co.uk or write to Mark King, Your Problems, The Observer, Kings Place, 90 York Way, London N1 9GU. Include a phone number.